The present invention relates to overvoltage detection, and in particular to a system and method for controlled overvoltage detection.
Electric power systems, such as those on an aircraft, are susceptible to overvoltage conditions. An overvoltage condition exists when the voltage applied to a load, for example, is larger than a voltage the load is rated to handle. These conditions may occur, for example, due to a lightning strike. If the voltage is large enough, or exists for long enough, permanent damage can be incurred by the system. Therefore, it is necessary to detect overvoltage conditions so that they may be handled prior to damaging the circuit.
Aside from unpredictable occurrences such as lighting strikes, overvoltage conditions may occur due to predictable events such as removing or adding power to a load. These events can create transients within the power system that do not pose a threat to the system if their duration is short. These transients have a natural recovery time, and it is desirable to not indicate an overvoltage condition if the system recovers within this natural recovery time. Therefore, timed trip overvoltage detection has been implemented to accommodate these expected short transients.
Timed trip overvoltage detection has been accomplished in the past using a simple resistor-capacitor (RC) circuit. A sensed voltage is input to the RC circuit, and the voltage across the capacitor is compared to a reference voltage. This reference voltage is generally set to a voltage that will cause an overvoltage indication at 5τ, where τ=R*C (i.e., when the capacitor is fully charged). Therefore, the higher the sensed voltage, the faster the capacitor will reach that trip threshold. This creates a “trip curve” where the necessary voltage to indicate an overvoltage fault decreases with time. The rate of change of the trip curve is equal to the rate of change of the voltage across the resistor of the RC circuit, which is approximately 1/e^(t/τ). Because of this, using solely an RC circuit greatly limits control of the characteristics of the timed trip curve.